This is quoted from HowStuffWorks.Com
What is Work?
The work we are talking about here is work in the physics sense. Not home work, or chores, or your job or any other type of work. It is good old mechanical work.
Work is simply the application of a force over a distance, with one catch -- the distance only counts if it is in the direction of the force you apply. Lifting a weight from the ground and putting it on a shelf is a good example of work. The force is equal to the weight of the object, and the distance is equal to the height of the shelf. If the weight were in another room, and you had to pick it up and walk across the room before you put it on the shelf, you didn't do any more work than if the weight were sitting on the ground directly beneath the shelf. It may have felt like you did more work, but while you were walking with the weight you moved horizontally, while the force from the weight was vertical.
Your car also does work. When it is moving, it has to apply a force to counter the forces of friction and aerodynamic drag. If it drives up a hill, it does the same kind of work that you do when lifting a weight. When it drives back down the hill, however, it gets back the work it did. The hill helps the car drive down.
Work is energy that has been used. When you do work, you use energy. But sometimes the energy you use can be recovered. When the car drives up the hill, the work it does to get to the top helps it get back down. Work and energy are closely related. The units of work are the same as the units of energy, which we will discuss later
Work can be done in the horizontal plane, aslong as the force you are considering is also in that plane or has a component in that plane (i.e. i force applied diagonaly to a box moxing horizontally along the ground).
We have to remember that physics is uses many assumption. For most purposes these assumptions are valid.
I.e we normally take Gravity = 9.81m/s/s but really it changes (slightly) depending on location.
The asumption about running and walking is valid. Assuming everything is constant except velocity and time. But in reality this isnt quite true. Its just a basic assumption.
I can walk alot further in a day that i could run. given the same food input.
DD90 - you make a very good point. The human body is not purely a mechanical system.